This application relates generally to audio level detection circuits for use in audio signal processing. More particularly, it concerns energy level detection method and apparatus that preserve the tonal balance of an audio input signal by detecting the spectral energy of the audio input signal over its dynamic frequency range and producing a gain control signal that is generally equally responsive to the high- and low-frequency components of the audio input signal.
Prior art audio level detectors ignore the negative amplitude versus frequency slope that is characteristic of audio signals, e.g. speech or musical program material. Conventionally, an audio signal level detector simply rectifies an audio input signal and integrates, or smooths, the rectified output to produce a direct current (DC) gain control signal--for use by a variable gain control device--the amplitude of which is directly proportional over the audio frequency spectrum to the amplitude of the audio input signal. Because of the characteristic nature of an audio input signal, in which lower frequencies tend to be of higher amplitude and higher frequencies tend to be of lower amplitude (in accordance with the so-called "pink noise" curve), the output of the variable gain device that processes the audio input signal is of constant amplitude, irrespective of the frequency of the audio input signal. One problem with variable gain devices so-controlled is a resultant shift of the tonal balance and a reduction of the timbre of the audio input signal so-processed to produce a gain-controlled audio output signal.
It is a principal object of the present invention to provide an audio signal energy, rather than amplitude, level detection circuit that preserves the tonal balance and timbre of the audio signal processed by a variable gain device controlled by the level detection circuit
It is also an object of the invention to provide such a circuit with a filter having a predefined, positive, amplitude versus frequency slope characteristic over the audio frequency range.
An important object of the invention is to provide such a filter circuit the amplitude versus frequency slope in decibels per decade (db/decade) of which is approximately equal in magnitude to that of a characteristic audio input signal and of opposite sign.
Another important object of the invention is to provide such a circuit in a simple, easily manufactured configuration that contains a minimum of components.
Yet another object is to provide such an energy level detection circuit that is straightforwardly incorporated into existing audio signal processing circuits and systems.
In brief summary, the invention in its preferred embodiment comprises an audio input signal terminal having series-connected thereto a high-pass filter having an approximately +10db/decade slope substantially over the audio frequency range and a (preferably signal-smoothing) rectifier the output of which is connectable to the control input of a variable gain device through which the audio signal may be processed. The beneficial result of high-pass filtering the audio input signal before rectification (and smoothing) is that a control signal having an amplitude that is generally constant, despite the audio inputs signal's decreasing amplitude at higher frequencies, is presented to the variable gain control device. Illustrative high-pass filters include passive and active, half-order, five-stage, +10db/decade circuits.